Influence of boron on structure and oxidation behavior of graphite fiber, P120 Original Research Article

P120 fibers, derived from mesophase pitch, were substitutionally doped with boron in the concentration ranges of 200–4600 ppm. An oxidation study was carried out in dry air at 973, 1023, and 1073 K at 95 kPa. Boron is preferentially positioned into the less disordered core region and in the external surface area (skin) of the fiber. Upon oxidation these regions are preferentially protected. Oxidation rates decreased by a factor up to 3, varying with boron concentration, burn-off level and oxidation temperature. The activation energy of oxidation increased from 151 kJ/mol for heat-treated P120 fibers to 180 kJ/mol for fibers with 3300 ppm B, then decreased to 122 kJ/mol for fibers containing 4600 ppm of boron. The observed decrease in oxidation rate is directly attributed to the location and concentration of boron. Boron doping inhibits oxidation by blocking specific active sites. It is proposed that 1000 ppm B as a threshold concentration at which the electronic, chemical, and physical (structural) behavior could be modified.